1. Technical Field of the Invention
The invention relates to a method of cleaning a chemical vapor deposition system(referred to in the present patent specification as “CVD system”).
2. Description of Related Art
It is known to use high-temperature polysilicon-type TFTs (thin-film transistors) and low-temperature polysilicon-type TFTs in methods of manufacturing liquid crystal displays.
In order to obtain high-quality oxide films in the manufacturing methods using high-temperature polysilicon-type TFTs, quartz substrates which could withstand high temperatures of 1000° C. or more are used.
In contrast, in the manufacture of low-temperature polysilicon-type TFTs, it is necessary to carry out film deposition in a low-temperature environment (for example 450° C. or less) because a glass substrate which is customary for TFTs is used. Methods for manufacturing liquid crystal displays using low-temperature polysilicon-type TFTs have the advantage that they do not require special substrates to be used. Such methods have been put into practice in recent years and their production volume is continuing to expand.
In the manufacture of liquid crystal displays using low-temperature polysilicon-type TFTs, plasma CVD is used when a silicon oxide film is deposited as a gate insulator film at low temperature. When silicon oxide film is deposited by plasma CVD, silane, tetraethoxysilane (TEOS) and the like are used as typical materials in gas form.
If silane, or the like, is used as the material in gas form and silicon oxide film is deposited by means of plasma, in the conventional plasma CVD system, silicon oxide film is deposited on the surface of a substrate by introducing the material in gas form and oxygen, or the like, into the space in front of said substrate, generating plasma in a gas mixture comprising the material in gas form and the oxygen or the like and exposing the substrate to said plasma.
The conventional plasma CVD systems are configured in such a way that the material in gas form is supplied directly into the plasma which is generated inside the plasma CVD system. For this reason, with the configuration of conventional plasma CVD system, there is a problem that the high-energy ions are injected from the plasma present in the space in front of the substrate onto the film-depositing face of the substrate and they damage the silicon oxide film and degrade the properties of the film. Furthermore, as the material in gas form is fed directly into the plasma, particles are produced by violent reaction between the material in gas form and the plasma, and as a result the yield is reduced.
In order to solve the above mentioned problems, an attempt to improve the CVD device of the remote plasma type is disclosed in Japan Patent Application Serial Number H11-157692.
The CVD device disclosed in the above mentioned the patent application, Serial Number H11-157692, produces active seeds (radicals) by generating plasma inside a vacuum vessel, carries out the film-deposition processing on a substrate, accommodated inside said vacuum vessel, by means of these active seeds and material in gas form.
That is to say, an electrically conductive partition plate which divides the interior of said vacuum vessel into two chambers is provided in said vacuum vessel. The interior of one of these two chambers is formed as a plasma-generating space in which high-frequency electrode are arranged, and the interior of the other chamber is formed as a film-deposition processing space in which a substrate-holding mechanism on which a substrate is mounted is arranged. A plurality of through-holes which are made to pass from the plasma-generating space to the film-deposition processing space are formed in this electrically conductive partition plate. Furthermore, this electrically conductive partition plate has an interior space which is divided off from the plasma-generating space and communicates with the film-deposition processing space via a plurality of diffusion holes. The system is configured in such a way that the material in gas form is supplied to the interior space of this electrically conductive partition plate from the outside and fed into said film-deposition processing space through said plurality of diffusion holes. The active seeds which are generated in said plasma-generating space are fed into the film-deposition processing space through the plurality of through-holes formed in said electrically conductive partition plate and film processing is performed on said substrate in film-deposition processing space.
In said CVD system disclosed in Patent Application Serial Number H11-157692, the plurality of through-holes which are made to pass from said plasma-generating space and are provided in said electrically conductive partition plate to said film-deposition processing space are formed to satisfy the condition uL/D>1 when the gas flow velocity inside said through-holes is u, the effective length of the through-holes is L and the coefficient of mutual gas diffusion is D.
As the plasma-generating space and film-deposition processing space are separated by means of the electrically conductive partition plate in said CVD system proposed in Patent Application Serial Number H11-157692, the device is configured in such a way that the processing surface of the substrate which is arranged in the film-deposition processing space is not exposed to the plasma. In addition, a plurality of through-holes which are made to pass from the plasma-generating space to film-deposition processing space are formed in the electrically conductive partition plate. However, because these through-holes are formed so as to satisfy the above mentioned condition, the material in gas form which is fed into the film-deposition processing space is prevented from diffusing back into the plasma-generating space.
It is to be noted that in Patent Application Serial Number H11-157692, a CVD system is proposed which is formed in such a way that said plurality of diffusion holes also fulfill the above mentioned condition placed on the through-holes, in order to prevent the active species fed into the film-deposition processing space from diffusing back into the interior space of the partition plate.
In fact, Patent Application Serial Number H11-157692 discloses a CVD system in which plasma is generated between the high-frequency electrode and the lower face part of the upper part of the vacuum vessel and in the space which is bounded by the high-frequency electrode and the partition wall comprised of vacuum vessel which makes up the CVD system and the electrically conductive partition plate, both of which are at ground potential. Further more, the variation of the above mentioned CVD system is disclosed in which, the high frequency electrodes are installed in upper positions in the plasma-generating space and plasma electrical discharge is produced between the high-frequency electrode and the electrically conductive partition plate.
Generally, there are problems common to CVD systems that when films continue to be deposited, they are also deposited on the substrate-supporting elements and the interior wall of the film-depositing chamber and the like. When they drop off onto the substrate during film deposition as particles, they cause to be disconnect circuits of the wiring and result in the reduction of the yield of manufactures products.
For this reason, apart from the film-depositing process, optimum cleaning is carried out after processing the prescribed number of substrates, said cleaning being performed using particular cleaning gases according to differences in the plasma-forming method and structures and compositions of the deposited materials. The cleaning of this type of CVD device is an important process, as is the film-deposition process in the implementation of stabilized operation of the CVD system without exposing the interior of the depositing chamber to the atmospheric ambient.